This invention relates to the purification of gas streams. In one of its aspects this invention relates to the removal of small amounts of mercury from a sulfur-free natural gas. In another of its aspects this invention relates to the precipitation of mercury by contact with suitable sulfur-containing agents. In yet another of its aspects this invention relates to the absorption of hydrogen sulfide using an amine absorption agent. In its basic concept this invention relates to an integrated process for treating a gas stream with a precipitating agent to remove mercury and further treating the gas stream with an absorption agent to control the amount of precipitating agent left in the gas stream.
Most raw natural gas contains hydrogen sulfide in concentrations varying from a few parts per million to 20 mole percent or more. This sulfur content is the result of natural gas deposits being associated with sulfur or sulfur-containing minerals or with the natural gas being formed in the presence of sulfur-containing organic matter. Sulfur-containing natural gas does not contain mercury because mercury that had been present in the reservoir would have been converted to a non-volatile sulfide.
There are, however, some natural gas deposits that are sulfur-free. In recent years it has been discovered that many sulfur-free natural gas deposits contain small amounts of mercury, usually in the range of about 1 part per million or less. The presence of even this small amount of mercury poses a potential health hazard under some circumstances. There is, also, an ever-present potential safety hazard in using this gas in cryogenic processing plants where the mercury can amalgamate with aluminum in processing equipment eventually weakening the metal and leading to a leakage or rupture.
It has now been discovered that mercury can be removed essentially quantitatively by injecting at least a stoichiometric amount of hydrogen sulfide into a natural gas stream to precipitate sulfides of mercury. Since from a practical standpoint it is necessary to inject an excess of hydrogen sulfide into the gas stream over that which will stoichiometrically combine with the mercury, further treatment of the natural gas stream is necessary both for controlling the sulfur content of the treated gas and to remove the precipitated sulfides of mercury.
It is therefore an object of this invention to provide a process for treating gas streams to lower the mercury content of the gas. It is also an object of this invention to provide a treated gas stream from which mercury bas been removed by treatment with hydrogen sulfide and which contains a minimal amount of sulfur treating agent. It is a further object of this invention to provide a process by which mercury can be removed from a natural gas stream by treating with hydrogen sulfide with the retention of a minimal amount of hydrogen sulfide in the natural gas stream.
Other aspects, concepts, objects and the various advantages of this invention will become apparent upon reading the specification and the appended claims.